It has been well known from old that a monocyclic aromatic hydrocarbon formaldehyde resin is obtained by allowing a monocyclic aromatic hydrocarbon such as xylene, etc. and formaldehyde to react with each other in the presence of an acid catalyst such as sulfuric acid, etc. (see, for example, Non-Patent Document 1). Also, it has been known that a polycyclic aromatic hydrocarbon formaldehyde resin is obtained from formaldehyde as well as a dimethylnaphthalene isomer mixture or a mixture of a dimethylnaphthalene isomer mixture and methylnaphthalene (see, for example, Patent Document 1).
In recent years, because of an increase of awareness concerning the environment, there is desired a material which is rich in flame retardancy even without using a phosphorus based flame retardant or a bromine based flame retardant. In connection therewith, it is carried out to introduce a polycyclic aromatic skeleton into a resin serving as a raw material. However, in general, since a polycyclic aromatic compound such as naphthalene, etc. is obtained from cokes such as coal, etc., undesirable impurities such as a sulfur-containing compound, a nitrogen-containing compound, etc. are incorporated thereinto. Thus, the incorporation of a sulfur compound or a nitrogen-containing compound into a resin which is obtained using it as a raw material is unavoidable.
Also, though there may be the case where an aromatic hydrocarbon formaldehyde resin is used as it is, in order to use it for a variety of applications such as an application for thermosetting resin material, etc., in many cases, the aromatic hydrocarbon formaldehyde resin is further subjected to a modification reaction with phenols, carboxylic acids, polyols, etc. For that reason, the resin is preferably a polyfunctional resin. Here, among hydrogen atoms directly bonded on an aromatic ring of an aromatic hydrocarbon as a raw material of the resin, a mean value of the number of hydrogen atoms substituted by a reaction during the production of a resin (a mean value of the number of substituted hydrogen atoms per one aromatic ring) can be utilized as an index showing polyfunctionality of the resin. In measuring the obtained resin by means of 1H-NMR, in the case where a xylene is used as a raw material, the mean number of substituted hydrogen atoms means a numerical value calculated utilizing an integrated value of methyl protons in the vicinity of from 1.8 to 2.6 ppm and an integrated value of protons directly bonded on the aromatic ring in the vicinity of 6.9 ppm; and in the case where a methylnaphthalene based compound is used as a raw material, the mean number of substituted hydrogen atoms means a numerical value calculated utilizing an integrated value of methyl protons in the vicinity of from 2.3 to 3.2 ppm and an integrated value of protons directly bonded on the aromatic ring in the vicinity of from 6.8 to 8.2 ppm.
However, in the case where naphthalene or monomethylnaphthalene is used as a raw material, it is difficult to obtain a polyfunctional naphthalene formaldehyde resin by a usual method so that it was necessary to perform a special reaction such as an interface reaction (see Patent Documents 2 and 3). Also, even in the case where a dimethylnaphthalene is used as a raw material, it has become clear that there may be the case where a polyfunctional resin is not obtained.
[Non-Patent Document 1] Kobunshika (Polymerization), supervised by IMOTO, Minoru, Kagaku Kogyo K.K., published in February 1966
[Patent Document 1] JP-A-54-86593
[Patent Document 2] JP-A-61-228013
[Patent Document 3] JP-A-11-92543